Ingot moulds

ABSTRACT

A cast iron ingot mould incorporates graphite in a matrix which has a predominantly pearlitic structure in the region surrounding the mould cavity and a predominantly ferritic structure in the outer region surrounding the pearlitic region. In one embodiment of the invention the ingot mould is initially cast with a matrix consisting 85% by volume of pearlite and graphite in 85% vermicular form. The mould is heated in a furnace to a temperature well above the A1 temperature so that the outer section of the mould is annealed to ferrite. At the same time the mould cavity is cooled e.g. by cold air so as to cool the inner region of the mould and maintain a pearlitic structure in this region.

United States Patent [1 1 Thomas et al.

[ Jan. 14, 1975 INGOT MOULDS [75] Inventors: William Ormond Thomas,

Glamorgan; Cyril Dawson Harle, Newport, both of Wales [73] Assignee: British Steel Corporation, London,

England [22] Filed: Feb. 19, 1974 [21] Appl. No.: 443,516

[30] Foreign Application Priority Data Feb. 20, 1973 Great Britain 8367/73 [52] U.S. Cl 148/138, 148/35, 148/139, 249/135 [51] Int. Cl..... B28b 7/00, C22c 37/00, C22c 37/04 [58] Field of Search 148/138, 139, 35; 249/135 [56] References Cited UNITED STATES PATENTS 1,762,098 6/1930 Kauffman 249/135 l/1932 Ramage ..249/l35 5/1960 Guenzi 249/135 [57] ABSTRACT A cast iron ingot mould incorporates graphite in a ma trix which has a predominantly pearlitic structure in the region surrounding the mould cavity and a predominantly ferritic structure in the outer region surrounding the pearlitic region. In one embodiment of the invention the ingot mould is initially cast with a matrix consisting 85% by volume of pearlitc and graphite in 85% vermicular form. The mould is heated in a furnace to a temperature well above the A temperature so that the outer section of the mould is annealed to ferrite. At the same time the mould cavity is cooled e.g. by cold air so as to cool the inner region of the mould and maintain a pearlitic structure in this region.

25 Claims, 1 Drawing Figure INGOT MOULDS This invention relates to cast iron ingot moulds and is particularly concerned with cast iron ingot moulds having an improved matrix structure.

It is an object of the present invention to provide cast iron ingot moulds combining the advantages of prior art moulds, that is to say, having a combination of such properties as ductility, thermal conductivity, tensile strength, and resistance to oxidation and grain growth of the order of the best obtainable in existing moulds.

According to one aspect of the present invention a cast iron ingot mould incorporates graphite in a matrix which has a predominantly pearlitic structure in the region surrounding the mould cavity and a predominantly ferritic structure in the region surrounding the pearlitic region.

According to a further aspect of the present invention a process for producing a cast iron ingot mould comprises heat-treating a mould having a matrix incorporating graphite so that an inner region of the mould surrounding the mould cavity has a matrix which is predominantly pearlitic in structure and an outer region of the mould surrounding the inner region has a matrix which is predominantly ferritic in structure.

Previously cast iron ingot moulds have been produced with the ferrite/pearlite ratio of the matrix being substantially constant throughout the mould. The ingot mould of the present invention combines the advantages of both pearlite and ferrite in that the predominantly pearlitic region enables the mould to be strongly resistant to grain growth and oxidation while retaining toughness and strength and the predominantly ferritic region imparts to the mould a higher thermal conductivity whereby the mould is less susceptible to cracking caused by thermal shock.

In one embodiment of the invention the heat treatment comprises heating the outer surface of a mould having a matrix consisting predominantly of pearlite to such a temperature and for such a period that the outer region has its matrix converted to a predominantly ferritic structure, maintaining the inner surface of the mould during the heating at a temperature selected so that the inner region of the mould adjacent the outer region has a matrix remaining predominantly pearlitic in structure and cooling the mould from the temperature at a rate selected to retain the ferritic structure of the outer region.

The outer surface of the mould should be heated to above the A temperature so that sufficient heat is conducted through the outer surface to anneal pearlite in the outer region to ferrite. Preferably, the inner surface of the mould surrounding the cavity is force cooled to maintain it during heating at a temperature selected so that the inner region of the mould adjacent the outer region remains as a predominantly pearlitic structure. Suitably this temperature is below the A, temperature. Conveniently, the ingot mould is placed in a furnace at a temperature well above the A temperature while at the same time a cooling gas, such as air, is injected into the cavity region of the mould in order to cool this region and maintain it below the A temperature.

In an alternative embodiment of the invention, the heat treatment comprises selectively producing, between the inner surface and the outer surface of the mould, differential cooling from a temperature at which the matrix of the mould consists predominantly of ferrite so that an inner region of the mould surrounding the mould cavity has a matrix which is predominantly pearlitic in structure and an outer region of the mould has a matrix which remains predominantly ferritic in structure.

In this case the temperature is preferably above the A, temperature.

Since thermal cracking is one of-the main causes for failure of cast iron ingot moulds during service, the ferritic region should preferably be thicker than the pearlitic region so that the mould has good thermal conductivity. Suitably, the ferritic region is at least three times as thick as the pearlitic region.

While it is desirable that the pearlitic region should contain as little ferrite as possible, preferably less than 15% it has been found that up to 40% of ferrite can be tolerated in the pearlitic matrix without seriously affecting the desired properties of the pearlitic region. Similarly, the ferritic region can tolerate up to 40% pearlite without seriously affecting its properties, although it is preferable if the pearlite'content is less than 15%.

The graphite may be present in any convenient form, but is preferably vermicular or spheroidal. In this case the mould has good tensile strength, ductility and also low thermal conductivity. The formation of vermicular or spheroidal graphite is promoted, suitably by the presence in the molten iron from which the mould is produced of selected quantities of any one or more of the elements magnesium, cerium and titanium. In addition the formation of vermicular graphite may be promoted by the presence in the molten iron from which the mould is produced of a suitable quantity of soluble nitrogen, that is, nitrogen uncombined with'stable nitride forming elements. Suitably, the soluble nitrogen content exceeds 0.005% by weight and preferably lies within the range 0.005 to 0.015% by weight.

The invention will now be particularly described with reference to the accompanying examples, the accompanying drawing showing a sectional side view in elevation of a cast iron ingot mould being treated by the process described in Example 1 so as to have a matrix in accordance with the present invention.

EXAMPLE 1 A cast iron produced by conventional methods had the composition shown in the accompanying table:

A micro-examination of the ingot mould revealed that the matrix contained pearlite, the remainder being ferrite except for incidental inclusions while the graphite was 85% of vermicular form.

Referring to the drawing, the cast iron ingot mould l was placed within an electrically heated furnace 2 on a cast iron slab 3 of annular shape. The top of the mould was covered by a further castiron slab 4 of square section so that the mould cavity 5 was shieled from the furnace atmosphere. Slab 4 was fitted with a thermocouple 6 so that the temperature within the cavity 5 could be recorded. An air inlet in the form of a pipe 7 was located at the base of the furnace 2, and was directed upwardly into the cavity 5 through the centre of the annular slab 3. Air outlet holes 8 and 9 were formed in slab 4 when cast and were located adjacent to and on either side of thermocouple 6.

The ingot mould l was heated above the A temperature whilst in the furnace 2 to anneal the outer region of the mould to ferrite while at the same time cold air was blown through the pipe 7 so as to cool the region 10 surrounding the mould cavity and thereby prevent this region from being annealed to form ferrite. The temperature to which the mould l is heated depends on mould section and composition and in this case was 750C. The temperature within the mould cavity is also variable, but in this case was 600C.

The heating was continued for 20 hours until the required amount of ferrite had been formed on annealing by heat diffusion through the mould surface.

The heating was stopped when it was judged that the ferrite region was three times as thick as the pearlitic region. In the drawing the closely spaced crosshatching designates the pearlitic region while the less densely spaced cross-hatching designates the ferritic region. The mould 1 was then allowed to cool in the furnace for 12 hours and was then removed from the furnace 2 and allowed to cool naturally in air.

EXAMPLE 2 Molten cast iron of the composition shown in Example l was cast into a suitable sand mould well known in the art so as to solidify and form a cast iron ingot mould. After solidification the cast iron mould was allowed to cool in the sand mould to a temperature about 150C above the A temperature e.g. 850C so that the matrix consisted predominantly of ferrite (80%). The sand enclosed by the ingot mould cavity was then removed so as to produce differential cooling between the inner surface and the outer surface of the mould. In this way the inner surface cooled more rapidly than the outer surface and a predominantly pearlite matrix was produced in the inner region surrounding the cavity. After the outer surface of the ingot mould had cooled below the A temperature to say 600C, the ingot mould was removed from the remainder of the sand mould and was allowed to cool in air at room temperature. The ingot mould was found to have a ferrite region four times as thick as the pearlite region. If a thicker pearlitic region is desired cast iron chills well known in the art can be inserted into the mould cavity after the sand has been removed.

lngot moulds produced in accordance with the present invention are found to be more resistant to thermal stress than prior art moulds and are much less liable to crack. In addition the pearlite region surrounding the mould cavity ensures that the mould has high strength in order to resist mechanical shock caused by molten metal striking the walls of the mould cavity during pouring. The pearlite region also ensures that the ingot mould has increased resistance to inner surface oxidation and subsequent crazing which gives sticking and wall erosion. In this respect the ingot moulds of the present invention combine the advantages of both a ferritic and a pearlitic matrix.

We claim:

1. A cast iron ingot mould incorporating graphite in a matrix which has a predominantly pearlitic structure in the region surrounding the mould cavity and a predominantly ferritic structure in the region surrounding the pearlitic region.

2. A mould according to claim 1 in which the ferritic region is thicker than the pearlitic region so that the mould has an improved thermal conductivity.

3. A mould according to claim 2 in which the ferritic region is at least three times as thick as the pearlitic region.

4. A mould according to claim 1 in which the pearl itic region contains up to 40% by volume of ferrite.

5. A mould according to claim 4 in which the pearlitic region contains less than 15% by volume of ferrite.

6. A mould according to claim 1 in which the ferritic region contains up to 40% by volume of pearlite.

7. A mould according to claim 6 in which the ferritic region contains less than 15% by volume of pearlite.

8. A mould according to claim 1 in which the graphite is vermicular or spheroidal.

9. A mould according to claim 8 in which the molten iron used to make the mould contains selected quantities of any one or more of the elements magnesium, ce-

rium and titanium to promote the formation of vermicular graphite.

10. A mould according to claim 8 in which the molten iron used to make the mould contains a suitable quantity of soluble nitrogen to promote the formation of vermicular graphite.

11. A mould according to claim 10 in which the content of soluble nitrogen exceeds 0.005% by weight.

12. A mould according to claim 11 in which the content of soluble nitrogen lies within the range 0.005 to 0.015% by weight.

13. A process for producing a cast iron ingot mould having a matrix incorporating graphite comprising heat-treating the mould so that an inner region of the mould surrounding the mould cavity has a matrix which is predominantly pearlitic in structure and an outer region of the mould surrounding the inner region has a matrix which is predominantly ferritic in structure.

14. A process according to claim 13 in which the heat-treatment comprises heating the outer surface of a mould having a matrix consisting predominantly of pearlite to such a temperature and for such a period that the outer region has its matrix converted to a predominantly ferritic structure, maintaining the inner surface of the mould during the heating at a temperature selected so that the inner region of the mould adjacent the outer region has a matrix remaining predominantly pearlitic in structure and cooling the mould from the temperature at a rate selected to retain the ferritic structure of the outer region.

15. A process according to claim 14 in which the outer surface of the mould is heated to above the A temperature so that sufficient heat is conducted through the outer surface to anneal pearlite in the outer region to ferrite.

16. A process according to claim 15 in which the inner surface of the mould is force cooled to maintain it during heating at a temperature selected so that the inner region of the mould adjacent the outer region retains a matrix which is predominantly pearlitic in structure.

17. A process according to claim 16 in which the temperature is below the A, temperature.

18. A process according to claim 14 in which the mould is placed in a furnace at a temperature well above the A temperature while at the same time a cooling gas, such as air, is injected into the cavity region of the mould in order to cool this region and maintain the inner region of the mould below the A temperature.

19. A process according to claim 13 in which the heat-treatment comprises selectively producing, between the inner surface and the outer surface of the mould, differential cooling from a temperature at which the matrix of the mould consists predominantly of ferrite so that an inner region of the mould surrounding the mould cavity has a matrix which is predominantly pearlitic in structure and an outer region of the mould has a matrix remaining predominantly ferritic in structure.

20. A process according to claim 19 in which the temperature is above the A temperature.

21. A process according to claim 13 in which the graphite is vermicular or spheroidal.

22. A process according to claim 21 in which the formation of vermicular or spheroidal graphite is promoted by the presence in the molten iron from which the mould is produced of selected quantities of any one or more of the elements magnesium, cerium and titanium.

23. A process according to claim 21 in which the formation of vermicular graphite is promoted by the presence in the molten iron from which the mould is produced of a suitable quantity of soluble nitrogen.

24. A process according to claim 23 in which the content of soluble nitrogen exceeds 0.005% by weight.

25. A process according to claim 24 in which the content of soluble nitrogen lies within the range 0.005 to 0.015% by weight. 

1. A cast iron ingot mould incorporating graphite in a matrix which has a predominantly pearlitic structure in the region surrounding the mould cavity and a predominantly ferritic structure in the region surrounding the pearlitic region.
 2. A mould according to claim 1 in which the ferritic region is thicker than the pearlitic region so that the mould has an improved thermal conductivity.
 3. A mould according to claim 2 in which the ferritic region is at least three times as thick as the pearlitic region.
 4. A mould according to claim 1 in which the pearlitic region contains up to 40% by volume of ferrite.
 5. A mould according to claim 4 in which the pearlitic region contains less than 15% by volume of ferrite.
 6. A mould according to claim 1 in which the ferritic region contains up to 40% by volume of pearlite.
 7. A mould according to claim 6 in which the ferritic region contains less than 15% by volume of pearlite.
 8. A mould according to claim 1 in which the graphite is vermicular or spheroidal.
 9. A mould according to claim 8 in which the molten iron used to make the mould contains selected quantities of any one or more of the elements magnesium, cerium and titanium to promote the formation of vermicular graphite.
 10. A mould according to claim 8 in which the molten iron used to make the mould contains a suitable quantity of soluble nitrogen to promote the formation of vermicular graphite.
 11. A mould according to claim 10 in which the content of soluble nitrogen exceeds 0.005% by weight.
 12. A mould according to claim 11 in which the content of soluble nitrogen lies within the range 0.005 to 0.015% by weight.
 14. A process according to claim 13 in which the heat-treatment comprises heating the outer surface of a mould having a matrix consisting predominantly of pearlite to such a temperature and for such a period that the outer region has its matrix converted to a predominantly ferritic structure, maintaining the inner surface of the mould during the heating at a temperature selected so that the inner region of the mould adjacent the outer region has a matrix remaining predominantly pearlitic in structure and cooling the mould from the temperature at a rate selected to retain the ferritic structure of the outer region.
 15. A process according to claim 14 in which the outer surface of the mould is heated to above the A1 temperature so that sufficient heat is conducted through the outer surface to anneal pearlite in the outer region to ferrite.
 16. A process according to claim 15 in which the inner surface of the mould is force cooled to maintain it during heating at a temperature selected so that the inner region of the mould adjacent the outer region retains a matrix which is predominantly pearlitic in structure.
 17. A process according to claim 16 in which the temperature is below the A1 temperature.
 18. A process according to claim 14 in which the mould is placed in a furnace at a temperature well above the A1 temperature while at the same time a cooling gas, such as air, is injected into the cavity region of the mould in order to cool this region and maintain the inner region of the mould below the A1 temperature.
 19. A process according to claim 13 in which the heat-treatment comprises selectively producing, between the inner surface and the outer surface of the mould, differential cooling from a temperature at which the matrix of the mould consists predominantly of ferrite so that an inner region of the mould surrounding the mould cavity has a matrix which is predominantly pearlitic in structure and an outer region of the mould has a matrix remaining predominantly ferritic in structure.
 20. A process according to claim 19 in which the temperature is above the A1 temperature.
 21. A process according to claim 13 in which the graphite is vermicular or spheroidal.
 22. A process according to claim 21 in which the formation of vermicular or spheroidal graphite is promoted by the presence in the molten iron from which the mould is produced of selected quantities of any one or more of the elements magnesium, cerium and titanium.
 23. A process according to claim 21 in which the formation of vermicular graphite is promoted by the presence in the molten iron from which the mould is produced of a suitable quantity of soluble nitrogEn.
 24. A process according to claim 23 in which the content of soluble nitrogen exceeds 0.005% by weight.
 25. A process according to claim 24 in which the content of soluble nitrogen lies within the range 0.005 to 0.015% by weight. 